Generators with open loop active cooling

ABSTRACT

A rotor body for a high-speed generator includes a rotor body with interior and exterior surfaces, a coolant inlet and outlet, and a rotor cooling path for actively cooling the rotor body. The coolant inlet and outlet extend between the interior and exterior surfaces. An interior segment of the rotor cooling path fluidly couples the coolant inlet and coolant outlet and is bounded by the rotor body interior surface. An exterior segment of the cooling path is bounded by the rotor body exterior surface and fluidly couples the coolant outlet an environment external to the rotor body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 61/866,780 filed Aug. 16, 2013 which isincorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to systems for generating electricalpower, and more particularly to high-speed generators having open loopactively cooled rotors.

2. Description of Related Art

Conventional space vehicles use on-board generators to provide power forvarious on-board devices. Developments towards the “more-electric” spacevehicle have led to increased power demands to support on-boardelectrical equipment employed in such space vehicle architectures.High-speed generators, e.g. generators having rotational components withspeeds approaching or exceeding 250,000 rotations per minute, are ofinterest in meeting these needs due to their relative compactness,efficiency, and high energy density.

One challenge with high-speed generators is that windage effects in thegap between the rotor and stator makes removing heat generated frommagnetic eddy losses and resistive heating difficult. This is because,at high rotational speeds, the frictional viscous drag of the fluidagainst the rotor surface heats the fluid sufficiently to make heattransfer less readily across the gap from the rotor to the stator. Italso can make maintaining the gap clearance difficult. Conventionalterrestrial generators, such as utility generators, employ closed looprotor and stator cooling systems for removing heat, often usingrelatively massive and complex cryogenic systems.

Conventional systems and methods for space vehicle power generation havegenerally been considered satisfactory for their intended purpose.However, there is a need for improved generators that are efficient andhave high power density. There also remains a need for generators thatare easy to make and use. The present disclosure provides solutions forthese needs.

SUMMARY OF THE INVENTION

A rotor for a high-speed generator includes a rotor body with aninterior surface and an opposed exterior surface, a coolant inlet andoutlet, and a rotor cooling path. An interior cooling path segment ofthe cooling path fluidly couples the coolant inlet with the outlet andis bounded by the rotor body interior surface. An exterior cooling pathsegment of the cooling path is bounded by the rotor body exteriorsurface. The exterior cooling path segment fluidly couples the coolantoutlet to the environment external to the rotor for actively cooling therotor body.

In certain embodiments, the rotor body defines coolant outlets disposedalong an axial length of the rotor body. The outlet can be bounded by asidewall extending between the interior and exterior surfaces of therotor body through a thickness of the rotor body. The sidewall canintersect a longitudinal axis of the rotor body obliquely such that thecoolant outlet is oriented toward a first end portion of the rotor body.The coolant outlet can be oriented toward a second end portion of therotor body. The sidewall can define a coolant outlet axis orientedradially outward and towards a direction of rotation of the rotor body.The coolant outlet axis can be oriented radially outward and in adirection opposing rotation of the rotor body.

In accordance with certain embodiments, the rotor body has a bafflecoupled to the rotor body inner surface that divides an interior cavityinto a plurality of coolant channels extending between the coolant inletand the coolant outlet. The coolant inlet can be configured to coolbearings supporting the rotor body using coolant traversing the coolantinlet. The rotor body can include a first and second coolant inletarranged on opposite ends of the rotor body.

It is contemplated that the coolant inlet is configured to fluidlycouple the rotor body to a cryogenic fuel supply. The coolant inlet canbe configured to fluidly couple the rotor body to a hydrogen, oxygen,xenon, or helium supply. The coolant outlet can be fluidly coupled to anexternal environment for open loop cooling of the rotor body.

A turbo-alternator includes a rotor as described above, a stator, and acooling path. The stator has an inner surface opposing the rotor bodyexterior surface and bounding the exterior cooling path segment of therotor cooling path. The cooling path fluidly couples to the rotorcooling path and includes a supply orifice and an exhaust orifice. Thesupply orifice is configured to fluidly couple with a pressurizedcoolant source for actively cooling the rotor body.

In certain embodiments, the cooling path includes a speed control valvefluidly coupled between the supply orifice and the coolant inlet of therotor body. The speed control valve can fluidly couple to the rotor bodythrough a first coolant inlet and a second coolant inlet. The first andsecond coolant inlets are arranged on opposite ends of the rotor body.

A generator includes an inner body and an outer body. The inner bodydefines a longitudinal axis, an interior surface and an opposed exteriorsurface, and a coolant inlet and outlet extending between the interiorand exterior surfaces of the inner body. The outer body is arrangedoutboard of the inner body. A cooling path defined by the inner andouter bodies includes an interior cooling path bounded by the inner bodyinterior surface and fluidly coupling the coolant inlet to the coolantoutlet, and an exterior exhaust path bounded by the inner body exteriorsurface and outer body interior surface for fluidly coupling the coolantoutlet to the external environment of the generator. The outer body isconfigured for rotation about the longitudinal axis of the inner body.

These and other features of the systems and methods of the subjectdisclosure will become more readily apparent to those skilled in the artfrom the following detailed description of the preferred embodimentstaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

So that those skilled in the art to which the subject disclosureappertains will readily understand how to make and use the devices andmethods of the subject disclosure without undue experimentation,preferred embodiments thereof will be described in detail herein belowwith reference to certain figures, wherein:

FIG. 1 is a cross-sectional perspective view of a turbo-alternatorgenerator;

FIG. 2 is a schematic view of the turbo-alternator of FIG. 1, showingthe coolant flow path of the turbo-alternator generator of FIG. 1;

FIG. 3 is a schematic cross-sectional view of a generator of theturbo-alternator of FIG. 1, showing the rotor body;

FIG. 4A is a schematic cross-sectional view an embodiment of a rotorbody, showing a rotor body baffle within the rotor body;

FIG. 4B is a schematic cross-sectional view another embodiment of arotor body, showing axially leaning coolant outlets oriented toward afirst rotor body end portion;

FIG. 4C is a schematic cross-sectional view yet another embodiment of arotor body, showing axially leaning coolant outlets oriented toward asecond rotor body end portion;

FIG. 5A is a schematic cross-sectional axial view of the rotor body ofFIG. 3, showing a plurality of coolant outlets circumferentiallydisposed about a circumference of the rotor body;

FIG. 5B is a schematic cross-sectional axial view of a rotor body,showing a tangentially oriented coolant outlet facing in a direction ofrotor body rotation; and

FIG. 5C is a schematic cross-sectional axial views of an embodiment of arotor body, showing a tangentially oriented coolant outlet facing in adirection opposite rotor body rotation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like referencenumerals identify similar structural features or aspects of the subjectdisclosure. For purposes of explanation and illustration, and notlimitation, a view of an exemplary embodiment of a rotor in accordancewith the disclosure is shown in FIG. 3 and is designated generally byreference character 100. Other embodiments of the rotor in accordancewith the disclosure, or aspects thereof will be described. The systemsand method described herein can be used in high-speed generators, suchas in turbo-alternators for spacecraft.

Referring to FIG. 1, a turbo-alternator 10 is shown. Turbo-alternator 10includes a generator 12, a turbine 14, and a coolant conduit 16. Turbine14 is mechanically coupled to generator 12 via an output shaft. Theoutput shaft mechanically couples turbine 14 through an intermediategear and an output gear, the output gear in turn rotating a shaft ofgenerator 12 to generate electrical power. As will be appreciated bythose skilled in the art, other coupling arrangements can be includedwithin the scope of the present disclosure.

Coolant conduit 16 extends between a supply orifice 18 and an exhaustorifice 20. Coolant conduit 16 fluidly couples turbine 14 and generator12 between supply orifice 18 and exhaust orifice 20, thereby placingeach in fluid communication with the others. As will be appreciated,coolant conduit 16 also includes suitably arranged and sized coolantconduit segments for interconnecting turbo-alternator elements betweensupply orifice 18 and exhaust orifices 20.

Supply orifice 18 fluidly couples turbo-alternator 10 to a coolantsupply 24 and is configured and adapted to receive a cryogenic fluid.The coolant includes a cryogen such as hydrogen, and more particularlyliquid hydrogen fuel from e.g., a space vehicle. The coolant can includegaseous, liquid, or a mixture of phases. Active cooling of generator isaccomplished by forcing coolant into generator 12 using a pump 22fluidly coupled between the coolant supply 24 and supply orifice 18.Active cooling of generator 12 may also be accomplished by fluidpressure within coolant supply 24 in a “blow down” type system whereinthe head of hydrogen in the supply tank accelerates the coolant toprovide pressure at supply orifice 18.

Typical space launch vehicles are propelled by combining hydrogen withan oxidizer in an exothermal chemical reaction to createhigh-temperature, high-pressure exhaust propellant gas. Hydrogen canalso serve as coolant for high-speed generators because of its lowdensity, low viscosity, high specific heat, and high thermalconductivity. Hydrogen is typically not used in space vehicles forgenerator coolant because (a) its value as fuel makes it too importantto use for open loop generator cooling, and (b) the cryogenic subsystemnecessary to scavenge and return hydrogen coolant to the fuel system isprohibitively massive for conventional space launch vehicles. However,embodiments of the rotor body described below allow for construction ofhigh-speed generators that require extremely limited amounts of coolant,making the advantages of open loop (e.g. a system where coolant isexhausted to the environment 26 external to the vehicle without returnto the vehicle fuel system or reuse) generator cooling with hydrogenfuel outweigh the disadvantages of consuming fuel for generator cooling.In embodiments, hydrogen consumed for open loop generator cooling duringthe vehicle lifespan is sufficiently small to warrant incorporating on aspace vehicle, and may be on the order of about 0.05% and 0.1% of thevehicle fuel supply.

Referring now to FIG. 2, turbo-alternator 10 is shown schematically.Turbo-alternator 10 includes a gear assembly 28 that mechanicallycouples turbine 14 to generator 12. The coupling can be a shaftconfigured to rotate a rotor of generator 12 using work extracted fromthe coolant. A speed control valve 30 fluidly communicates with supplyorifice 18 and generator 12 through coolant conduit 16 and is alsooperative to control the speed of turbine 14 and/or generator 12, suchas by throttling the coolant flow provided to turbo-alternator 10. Aswill be appreciated by those skilled in the art, the rotatable body canbe an inner body arranged within a fixed outer body. Alternatively, therotatable body can be a rotatable outer body arranged about a fixedinner body, e.g. an inside out generator.

Turbo-alternator 10 also includes a lube oil circuit with a lube oilpump 34, a lube oil bypass valve 36, and a lube oil filter 38interconnected by lube oil conduits 32 and 40. Lube oil conduits 32 and40 are disposed within the housing of turbo-alternator 10, andinterconnect oil pump 34, speed control valve 30, and gear assembly 28with turbine 14 and generator 12. Lube oil filter 38 is seriallydisposed between lube oil pump 34 and turbine 14, and operable to filterlube oil flowing between lube oil conduits 32 and 40. Lube oil bypassvalve 36 is coupled in parallel with lube oil filter 38 between oil pump34 and turbine 14, and is operable to bypass lube oil filter 38 in theevent of blockage or other malfunction. In embodiments, turbine 14 isoperable to function as an oil cooler by thermally transferring heatfrom lube oil circulated through turbine 14 into coolant flowing throughturbine 14, thereby extending the lifespan of lube oil circulating inthe system. As will be appreciated, respective elements of the lube oilsystem are interconnected by lube oil circuit conduit segments suitablysized and arranged to route oil as suitable for the application.

Referring now to FIG. 3, a rotor 100 for generator 12 is shown. Rotor100 includes a stator 50 circumferentially surrounding a rotor body 110.Stator 50 includes an interior surface 56 opposing rotor body 110. Rotorbody 110 includes an exterior surface 112 with rotor windings 60, afirst coolant inlet 114, and a coolant outlet 116. An interior rotorcooling path segment, indicated with arrows 118, is defined by aninterior surface 120, extends coolant inlet 114 with coolant outlet 116,and fluidly couples coolant inlet 114 with coolant outlet 116. Exteriorsurface 112 of rotor body 110 and interior surface of stator 50cooperatively define an axially extending circumferential gap 54. Gap 54forms an exterior cooling path segment, indicated with arrows 122,fluidly coupling coolant outlet 116 with the external environment.

Coolant inlet 114 can be a first coolant inlet 114 disposed on a firstend portion 124 of rotor body 110, e.g. the left hand side of rotor body110 as oriented in FIG. 3. Rotor body 110 can optionally include asecond coolant inlet 115 disposed on an opposed second end portion 128of rotor body 110, e.g. the right hand side of rotor body 110 asoriented in FIG. 3. Providing coolant through coolant inlets located onopposing ends of the rotor body allows for even flow of coolant throughrotor body 110 and permits balancing coolant flow.

Rotor body 110 is configured to rotate about its longitudinal axis byfirst and second shaft ends 132 and 133. First shaft end 132 supportsfirst end portion 124 of rotor body 110 and is supported by firstbearing 130. Second shaft end 133 supports second end portion 128 ofrotor body 110 and is supported by second bearing 131. Shaft ends 132and 133 respectively define axially extending channel coolant inletsfluidly coupling the interior of rotor body 110 (of generator 12) tocoolant supply 24 (shown in FIG. 1). First and second coolant inlets 114and 118 also respectively traverse first and second bearings 130 and 131and are configured to provide cooling to the bearings. This allows forconvectively removing heat generated by bearing friction using coolantsupplied to rotor body 110.

Coolant entering rotor body 110 through coolant inlet 114 traverses theinterior cooling segment of the rotor cooling path and removes heat fromrotor body 110 by convection. The heat can originate from resistiveheating of windings 60 (copper losses) or from eddy losses (iron losses)originating from changing magnetic flux within rotor body 110. First andsecond coolant inlets 114 and 118 can also include a nozzle configuredto spray coolant over interior surfaces of rotor body 110. This reducesthe volume of coolant necessary for cooling rotor body 110.

Coolant exiting rotor body 110 through coolant outlet 116 traverses theexterior cooling path segment, e.g. gap 54, of the rotor cooling pathand removes heat by advection. This heat can originate by windage, e.g.arising out of frictional drag of viscous fluid contacting movingexterior surfaces 112 or rotor body 110. The coolant exiting rotor body110 can also convect heat away from exterior surface 112 and interiorsurface 56 of stator 50. As will be appreciated by those skilled in theart, actively cooling rotor body 110 allows for increasing and/ordecreasing coolant flow rate in cooperation with generator operation tocontrol temperature within gap 54, thereby controlling the gap size bymanaging component expansion and contraction due to heating. As willalso be appreciated, using a coolant like hydrogen reduces windagelosses owing to the lower viscosity of the fluid. It also reduces thelikelihood of damage to rotor body windings 60 from corona discharges.

Referring now to FIG. 4A, a rotor 200 is shown including a rotor body210. Rotor body 210 defines an interior rotor body cavity 206 andincludes a baffle 202. Baffle 202 is coupled to rotor body interiorsurface 204 and divides rotor body cavity 206 into a first coolantchannel 208 and a second coolant channel 218. First and second coolantchannels 208 and 218 fluidly couple coolant inlet 214 with coolantoutlet 216. Baffle 202 improves cooling by (a) increasing the coolingsurface area, (b) increasing the residence time of coolant within rotorbody 210, and (c) increasing the mixing of the coolant within rotor body210. It also provides for cooling rotor body 210 with a single coolantinlet positioned on and end of rotor body 210. This simplifiesconstruction of rotor body 210.

In the illustrated embodiment, coolant enters rotor body 210 throughcoolant inlet 214, longitudinally traverses rotor body 210 from firstend portion 224 to second end portion 228, reverses direction in secondend portion 228, and traverses a portion of rotor body 210 a secondtime, thereafter exiting rotor body 210 through coolant outlet 216. Aswill be appreciated by those skilled in the art, baffle 202 can extendaxially through the rotor body interior. Alternatively, baffle 202 canextend circumferentially, contacting rotor body interior surface 204 andextending radially inwards. Baffle 202 can also define a spiral helixpattern as may be suitable for the heat removal requirements of aparticular generator. Other baffle patterns can be used to increase thecooling surface area and increase the residence time of coolant withrotor body 210.

Referring now to FIG. 4B, a rotor 300 is shown. Rotor 300 includes arotor body 310. Rotor body 310 is similar to rotor body 110 andadditionally includes a first coolant outlet 302, a second coolantoutlet 304, and a third coolant outlet 306. Coolant outlets 302, 304,and 306 are axially arranged at different longitudinal distances fromrespective end portions 324 and 328 of rotor body 310. First coolantoutlet 302 is axially closer to first end portion 324, third coolantoutlet 306 is axially closer to second end portion 328. This providesfor relatively uniform coolant distribution over exterior surface 312 ofthe rotor body for providing additional coolant flow to hot spots thatmay exist on the rotor body surface due to anomalies in rotorconstruction or during operational thermal transients. As will beappreciated, different numbers and pitches of coolant outlets can beselected as is suitable for a given application.

Coolant outlets 302, 304, and 306 can be defined by overlapping radialslots in adjoining rotor laminations. As will be appreciated by thoseskilled in the arts, the radial slots width, radial width and length canbe arranged to define radial, tangential, or circumferentially orientedcoolant outlets. Coolant outlets 302, 304, and 306 can also can also beformed, or optimized for example, to provide appropriate flow resistanceto coolant flowing though the coolant outlet. This constructionminimizes the electromagnetic penalty associated with discontinuities inthe rotor body as the magnetic flux lines are only intermittentlyinterrupted.

Referring now to FIG. 4C, a rotor 400 is shown. Rotor 400 includes arotor body 410. Rotor body 410 is similar to rotor body 110 andadditionally includes a coolant outlet 402 defined by sidewalls 404 and406. Sidewalls 404 and 406 extend through a thickness of rotor body 410between interior and exterior surfaces 420 and 412. Sidewalls 404 and406 define a coolant outlet axis that is oblique with respect to alongitudinal axis of rotor body 410 and oriented toward a second end 428or rotor body 410. As will be appreciated by those skilled in the arts,coolant outlet 402 can be axially leaning toward first end portion 424of the rotor body. Hybrid combinations are also possible. Such coolantoutlet orientations allow for balancing coolant flow within the rotorcavity.

Referring now to FIG. 5A, a rotor body 510 is shown. Rotor body 510 issimilar to rotor body 110, and additionally includes a coolant outlet502 defined by a sidewall 504 extending through rotor body 510. Rotorbody 510 can include one or more coolant outlets 502 disposedcircumferentially about a rotor body 510. Distributing coolant outletsabout the circumference of the rotor body provides for uniform coolingof the entire radial inner surface. It also prevents developing radialhot spots by uniformly distributing coolant through the interior of therotor body.

Referring now to FIG. 5B, a portion of a rotor body 610 is shown. Rotorbody 610 is similar to rotor body 110 and additionally includes acoolant outlet 602 defined by a sidewall 604. Sidewall 604 defines acoolant flow path E. Coolant flow path E intersects a line T tangentwith rotor body 610 to form an acute angle 606. Angle 606 is opposite adirection of rotation of rotor body 610. Orienting coolant outlet 602 ina direction opposite the direction of rotation (R) decreases the rate ofcoolant flow through the coolant outlet. This provides reduced rotorbody cooling near the outlet.

Referring now to FIG. 5C, a portion of a rotor body 710 is shown. Rotorbody 710 is similar to rotor body 110 and additionally includes acoolant outlet 702 defined by a sidewall 704. Sidewall 704 defines acoolant flow path F. Coolant flow path F intersects a line T tangentwith rotor body 710 to form an acute angle 706. Angle 706 is in thedirection of rotation (R) of rotor body 610. Orienting coolant outlet602 in the direction of rotation increases rate of coolant flow throughthe coolant outlet. This provides increased rotor body cooling near theoutlet.

Conventional generators are subject to heating from bearing friction,eddy losses due magnetic field change, and resistive heating fromcurrent flow within conductor windings. High-speed generators aresubject to frictional heating of fluid within the gap between the rotorand stator, e.g. windage losses. Embodiments of the rotor describedherein convect heat generated through eddy losses and resistive heatingby actively cooling the rotor body by forcing coolant through the rotorbody. Embodiments of the rotor described herein also advect away heatgenerated by windage by forcing coolant through the gap between therotor and stator. Embodiments of the rotor described herein furtherreduce the amount of heat generated by windage by purging the gapbetween the rotor and stator of air and replacing it with hydrogen.Since hydrogen conducts heat more effectively than air, heat is moreeffectively conducted from both the rotor and stator and into thehydrogen. Moreover, since windage losses are proportional to the densityof the medium occupying the air gap raised to the 2.5 power and toapproximately the square root of the medium's viscosity, the amount ofheat generated by windage is reduced. This allows for relatively smallamounts of hydrogen to cool the generator than were air, oxygen ornitrogen used for active cooling. As will be appreciated by thoseskilled in the art, actively cooling the rotor with a coolant likehydrogen provides a generator with greater rotor speed, efficiency,and/or power density than conventionally generator of similar size,geometry, and weight. As will also be appreciated, other gases readilyavailable coolants can be such as xenon, available in some types ofnuclear powered turbo-generator, or helium, available in some types ofhelium power auxiliary power units can be employed with similar effect.

The methods and systems of the present disclosure, as described aboveand shown in the drawings, provide methods and systems to generateelectrical power. Embodiments of the methods and systems describedherein can provide power using compact high-speed generators with activeopen loop cooling systems. While the apparatus and methods of thesubject disclosure have been shown and described with reference topreferred embodiments, those skilled in the art will readily appreciatethat changes and/or modifications may be made thereto without departingfrom the spirit and scope of the subject disclosure.

What is claimed is:
 1. A rotor for a high-speed generator, comprising: arotor body defining: an interior surface and opposed exterior surface; acoolant inlet and outlet extending between the interior and exteriorsurfaces; and a rotor cooling path including (i) an interior coolingpath segment bounded by the rotor body interior surface and fluidlycoupling the coolant inlet to the coolant outlet, and (ii) an exteriorcooling path segment bounded by the rotor body exterior surface andfluidly coupling the coolant outlet to an external environment for openloop cooling the rotor body; and a cryogenic fuel supply in fluidcommunication with the coolant inlet.
 2. A rotor as recited in claim 1,wherein the coolant inlet is configured to fluidly couple the rotor bodyto a cryogenic fuel supply.
 3. A rotor as recited in claim 1, whereinthe coolant inlet is configured to fluidly couple the rotor body to ahydrogen, oxygen, xenon, or helium supply.
 4. A rotor a recited in claim1, further comprising a baffle coupled to the rotor body inner surfaceand dividing an interior cavity of the rotor body into a plurality ofcoolant channels extending between the coolant inlet and the coolantoutlet.
 5. A rotor as recited in claim 1, wherein the rotor body definesa plurality of coolant outlets disposed along an axial length of therotor body.
 6. A rotor recited in claim 1, wherein the rotor bodydefines a plurality of coolant outlets disposed about a circumference ofthe rotor body.
 7. A rotor as recited in claim 1, wherein the coolantoutlet is bounded by a sidewall extending between the interior andexterior surfaces of the rotor body through a thickness of the rotorbody.
 8. A rotor as recited in claim 4, wherein the sidewall intersectsa longitudinal axis of the rotor body obliquely such that the coolantoutlet is oriented toward a first end portion of the rotor body.
 9. Arotor as recited in claim 4, wherein the sidewall intersects alongitudinal axis of the rotor body obliquely such that the coolantoutlet is oriented toward a second end portion of the rotor body.
 10. Arotor as recited in claim 4, wherein the sidewall defines a coolantoutlet axis oriented radially outward and towards a direction ofrotation of the rotor body.
 11. A rotor as recited in claim 4, whereinthe sidewall defines a coolant outlet axis oriented radially outward andopposite a direction of rotation of the rotor body.
 12. Aturbo-alternator, comprising: a rotor as recited in claim 1; a statorwith an inner surface opposing the rotor body exterior surface andbounding the exterior cooling path segment of the rotor cooling path;and a cooling path fluidly coupled to the rotor cooling path including asupply orifice and an exhaust orifice, wherein the supply orifice isconfigured to fluidly couple with a pressurized coolant source foractively cooling the rotor body, wherein the exhaust orifice is fluidlycoupled to an external environment for open loop cooling.
 13. Aturbo-alternator as recited in claim 12, wherein the coolant inlet isconfigured to cool bearings supporting the rotor body using coolant flowtraversing the coolant inlet.
 14. A turbo-alternator as recited in claim13, wherein the cooling path includes a speed control valve fluidlycoupled between the supply orifice and the coolant inlet of the rotorbody.
 15. A turbo-alternator as recited in claim 14, wherein the speedcontrol valve is fluidly coupled to the rotor body through a firstcoolant inlet and a second coolant inlet.
 16. A turbo-alternator asrecited in claim 15, wherein the first and second coolant inlets arearranged on opposite ends of the rotor body.
 17. A method of cooling agenerator, comprising: receiving hydrogen coolant from a vehicle fuelsupply at an inlet of a rotor body; flowing the hydrogen coolant throughthe rotor body along a coolant path including (i) an interior coolingpath segment bounded by the rotor body interior surface and fluidlycoupling the coolant inlet to the coolant outlet, and (ii) an exteriorcooling path segment bounded by the rotor body exterior surface; andexhausting the hydrogen coolant into the environment external to thevehicle by flowing the hydrogen coolant through a coolant outlet fluidlycoupling the exterior cooling path segment to the external environment.18. A rotor as recited in claim 1, further comprising a pump fluidlycoupling the cryogenic fuel supply to the coolant inlet.
 19. A rotor asrecited in claim 1, wherein the cryogenic fuel supply is fluidly coupledto the coolant inlet in blow down arrangement.